1. Field of the Invention
This invention relates to conductive polymer compositions for use as polymer thick film inks and methods of making said inks.
2. Background of the Invention
Thick film inks for use as resistors, connectors and other electrical components are known. These conventional inks normally exhibit ZTC behavior (zero temperature coefficient of resistance), i.e. they maintain a relatively constant resistance value over a temperature range of interest. The inks are usually applied via screen-printing or other means to a rigid substrate, e.g. alumina, beryllia, or glass; the rigid substrate serves to minimize any resistance change due to volume expansion of the substrate. Thick film inks usually comprise a conductive filler, e.g. graphite, ruthenium, or silver, in a glass, ceramic, or polymer binder. The binder acts as a matrix for the conductive filler and other components. Those inks in which the binder is a polymer are known as polymer thick film inks (PTF inks).
For some applications, e.g self-regulating heaters or circuit protection devices, materials exhibiting PTC behavior (positive temperature coefficient of resistance) are preferred. Conductive polymer compositions which exhibit PTC behavior and electrical devices comprising them are well-known. Reference may be made, for example, to U.S. Pat. Nos. 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,914,363, 4,017,715, 4,177,376, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,286,376, 4,304,987, 4,318,881, 4,330,703, 4,334,148, 4,334,351, 4,388,607, 4,400,614, 4,425,497, 4,426,339, 4,435,639, 4,459,473, 4,514,620, 4,520,417, 4,529,866, 4,534,889, 4,543,474, 4,545,926, 4,547,659, 4,560,498, 4,571,481, 4,574,188, 4,582,983, 4,631,392, 4,638,150, 4,654,511, 4,658,121, 4,659,913, 4,661,687, 4,667,194, 4,673,801, 4,698,583, 4,719,335, 4,722,758, 4,722,853, and 4,761,541, European Patent Publication No. 38,718 (Fouts et al), International Application No. PCT/US88/00592 (McMills et al.) filed Feb. 24, 1988, and copending, commonly assigned application Serial Nos. 818,846 (Barma) filed Jan. 14, 1986, now abandoned, 53,610 (Batliwalla et al.) filed May 20, 1987, now U.S. Pat. No. 4,777,351, 75,929 (Barma et al.) filed July 21, 1987, 115,089 (Horsma et al.) filed Oct. 30, 1987, now abandoned, 124,696 (Horsma et al.) filed Nov. 24, 1987, now abandoned in favor of three continuation applications, Ser. Nos. 455,715, 456,015, and 456,030, all filed Dec. 22, 1989, 150,005 (Fahey et al.) filed Feb. 4, 1988, now U.S. Pat. No. 4,780,598, 189,938 (Friel) filed May 3, 1988, now U.S. Pat. No. 4,882,466, 202,165 (Oswal et al.) filed June 3, 1988, 202,762 (Sherman et al.) filed June 3, 1988, now U.S. Pat. No. 4,910,389, 209,761 (Hughes et al.) filed June 22, 1988, now abandoned, 210,054 (McMills et al.) filed June 22, 1988, abandoned in favor of a continuation application, Ser. No. 407,595, filed Sept. 15, 1989, 219,416 (Horsma et al.) filed July 15, 1988 now U.S. Pat. No. 4,967,176, and 247,059 (Shafe et al.) filed contemporaneously with this application, the disclosures of which are incorporated herein by reference. The majority of these materials are not suitable for use as inks; rather they are melt-processed or sintered to produce self-supporting articles which have a thickness greater than about 0.002 inch (0.005 cm). The resulting articles may be inflexible and are generally unsuitable for configuration into the intricate or very thin shapes often desirable for use on flexible substrates or printed circuit boards.
U.S. Pat. No. 4,722,853 (Batliwalla et al.) discloses a method of applying a PTF ink to a substrate. For these inks, at room temperature the organic polymer binder is in the form of solid particles, i.e. not dissolved, and the solvent is a "latent" solvent, rather than a "true" solvent, for the binder.
U.S. Pat. No. 4,628,187 (Sekiguchi et al.) discloses a planar resistive heating element in which a conductive paste is screen-printed between an electrode pattern onto an insulating substrate. The conductive paste, which exhibits PTC behavior, comprises a mixture of ethylene/vinyl acetate copolymer, graphite, flame retardant, inert filler, and solvent. A phenolic resin layer deposited over the resistive element provides protection to the element and increases its resistance to thermal degradation when heated to a temperature greater than the melting point of the polymer binder.